This invention relates to mechanism for transmitting rotary drive between the engine and drive wheels of a vehicle and more particularly to drive mechanisms which provide for a selection of either a two wheel or a four wheel drive.
Many vehicles, including tractors, earth moving equipment, passenger vehicles and trucks, for example, have four wheel drive mechanism for use where high traction may be needed to move heavy loads or where the vehicle is operated under conditions where traction may be lost at one or more of the drive wheels because of characteristics of the surface over which the vehicle travels. To avoid unnecessary wear of four wheel drive components of the system and to minimize power losses, such vehicles are also usually capable of being operated in a two wheel drive mode when driving conditions permit this. In some vehicles, the drive transmitting mechanism also provides for a compound low drive ratio between the engine and wheels which is additional to the speed ranges realizable by shifting the vehicle transmission. In most instances the four wheel drive mode of operation is used at low vehicle speeds and the two wheel drive mode is employed at high speeds. Typical prior four wheel drive systems as used on tractors and earthworking equipment are disclosed in U.S. Pat. Nos. 2,971,595 and 2,946,238.
While prior four wheel drive systems may provide for more than one mode of operation as discussed above, there are still additional operational modes which would be highly desirable in order to increase the versatility of the vehicle for different operating conditions and in order to further reduce wear of the drive system. As pointed out above, a simple four wheel drive system of known form tends to force an equal division of torque between the front and rear axles. As a practical matter, it may be possible to obtain greater overall traction for the vehicle by delivering more torque to one axle because of unequal weight loads on the axles or for other reasons. Also, as pointed out above, it is generally considered undesirable to use the four wheel drive mode at higher vehicle speeds. One reason for this is that if there are slight differences in the front and rear axle drive paths, such a difference in the diameters of front and rear tires due to differences in the degree of inflation or the loading thereon, then a simple four wheel drive system forces some tire slip at one or both axles causing increased tire wear and increased stress on various other drive elements. These effects become more pronounced as the speed of the vehicle increases. However, there are operating conditions under which it would be desirable to employ four wheel drive at higher speeds if these undesirable effects can be alleviated. To accomplish this using known techniques would result in a very complex and costly drive system.